1. Field of the Invention
The present invention relates to an f-theta lens and a laser scanning unit including the same, and more particularly, to an f-theta lens having an improved rib structure protecting an effective portion of the f-theta lens, and a laser scanning unit including the same.
2. Description of the Related Art
A printing machine, such as a laser printer, includes a laser scanning unit that forms a latent electrostatic image corresponding to an image to be printed by radiating light onto a photosensitive medium. Such a laser scanning unit typically includes an optical system as shown in FIG. 1. Referring to FIG. 1, the optical system includes a laser diode 100 which emits laser light. A collimating lens 101 collimates the emitted laser light to be parallel with an optical axis. A slit 102 is attached to a front of the collimating lens 101 to confine transmission of the laser light. A cylinder lens 103 transmits the laser light to a surface of a polygon mirror 104. The polygon mirror 104 is rotated at a uniform rotational velocity by a motor 105, thereby scanning laser light transmitted through the cylinder lens 103. An f-theta lens 110 with a predetermined refractivity polarizes light reflected by the polygon mirror 104 in a main scanning direction, and compensates for an aberration, thereby focusing on a scanning surface. An image forming reflective mirror 107 reflects laser light transmitted through the f-theta lens 110 to form an image in the form of dots on a surface of a photoreceptor drum 108. An optical sensor 106 receives laser light reflected by a synchronous signal detection reflective mirror 109 and performs horizontal synchronization.
Japanese Patent Publication No. H5-188285 discloses an f-theta lens that reduces a deviation of contraction rates with ribs formed on opposite sides of a lens body of the f-theta lens that extend beyond a height of the lens body by the same height along the length of the f-theta lens. However, this f-theta lens has a problem of residual stress.
FIG. 2 is an enlarged side view of the f-theta lens 110 shown in FIG. 1 and cross-sections thereof, taken along lines A–A′ and B–B′. Referring to FIG. 2, the f-theta lens 110 includes a lens body 111 that focuses light incident from the polygon mirror 104 onto a scanning surface and ribs 112, formed on opposite sides of the lens body 111 to protect the lens body 111. For clarity of description, the lens body 111 and the ribs 112 are separately illustrated, but actually, the f-theta lens 110 is compose of a single body.
The lens body 111 is a lens having a wave shape and includes a concave portion 111b between two convex portions 111a. The concave portion 111b is in a central portion of the f-theta lens 110 between the convex portions 111a, which are thinner than the concave portion 111b. Light input and output surfaces of the f-theta lens 110, through which light enters and exits the lens body 111, respectively, are convex.
The ribs 112 extend beyond a peak of the lens body 111 by a predetermined height, e.g., 1 mm. The ribs 112 are higher than the surface of an effective portion of the f-theta lens 110 in order to prevent the lens body 111, and more particularly, an effective portion through which the laser light passes, from being scratched when the f-theta lens 110 is kept in a case.
The f-theta lens 110 is usually made of a plastic material using injection molding in order to increase productivity and reduce manufacturing cost. FIG. 3 illustrate a simulation of residual stress that occurs in the f-theta lens 110 due to a mold when the f-theta lens 110 is made of a plastic material using injection molding. Referring to FIG. 3, the f-theta lens 110 contracts toward its center at a predetermined rate within a mold 120 during a cooling process. A contraction rate of the f-theta lens 110 depends on a type of plastic material and manufacturing conditions such as an injection temperature and an injection molding pressure, but is consistent to within 1%. The contraction rate increases as molding thickness increases. When the f-theta lens 110 contracts after injection molding, the portions of the ribs 112 extending beyond the opposite sides of the lens body 111 is restricted by a mold wall 120a when a resin contracts toward the center of the f-theta lens 110, and thus a residual stress due to a bending moment remains in the edge of the lens body 111. As a result, a deviation in the shape of the lens body 111 is large at the opposite sides of the lens body 111.
FIG. 4 is a graph illustrating a deviation of residual stress of a lens body of a conventional f-theta lens after injection molding. The graph is fitted based on measured approximations. Referring to FIG. 4, the lens body 111 has a width of about 4 mm and has a large residual stress at the opposite sides of the lens body 111.